For coating cars, industrial machines, steel furniture, interior and exterior decorations for constructions, household electric appliances, plastics and others, coating compositions consisting essentially of melamine resins such as alkydmelamines, acrylic melamines and the like have heretofore been used.
However, the conventional coating compositions consisting essentially of such melamine resins are problematic in that they generate a large amount of harmful formalin when cured, and that the cured films have poor acid resistance and are therefore often corroded by acid rain. In particular, the problem with acid rain caused by recent air pollution has become more serious, and the conventional coating films are often etched, whitened or stained by acid rain.
In order to solve this problem, a coating composition has been proposed in Japanese Patent Application Laid-Open (JP-A) Hei-2-45577 and Hei-3-28650, in which are formed ester bonds that act as crosslinking points, through reaction of an acid with an epoxy group.
The coating film of the proposed composition has good acid resistance, but is still defective in that it is whitened in an accelerated weather resistance test. In addition, the low-temperature curability of the composition is poor, the coating film of the composition formed around car bodies is often scratched with brushes when the car bodies are washed with a car washer, and the storage stability of the composition is poor.
For finishing car bodies by top coating, generally employed is any of two types of metallic color finishing and solid color finishing.
For the metallic color finishing of those two, generally employed is a 2-coat 1-bake method of heating and curing coating composition, which comprises coating a substrate with a metallic base coat followed by overcoating it with a clear coat of an acrylic melamine resin composition in an wet-on-wet system.
For the other solid color finishing, generally employed is a 1-coat 1-bake method of using an alkydmelamine resin composition, in which the composition is heated and cured to form top coating. For recent severe requirements for the properties of coating films, including, for example, finish appearance, weather resistance, acid resistance, scratch resistance and stain resistance thereof, a method of overcoating a solid color-finished substrate with a clear coat of an acrylic melamine resin composition or the like has been proposed. However, even in the proposed method of overcoating the solid color finish with such a clear coat, coated articles having the properties of good acid resistance, good scratch resistance and good weather resistance could not as yet be obtained.
The present invention has been made in consideration of the current situation noted above, and its object is to provide a curable composition for top coating, which has good thermal curability to form good coating films having good acid resistance, weather resistance, waterproofness, scratch resistance and outward appearance, and which additionally has good storage stability. In particular, the composition forms excellent clear coats when used in forming car bodies. The invention further provides articles as coated with the composition.